1. Field of the Invention
The present invention relates to an automatic traction device to increase traction in snow, ice, or other slippery conditions, and more particularly to a means to protect the traction device from damage due to tire-roadway interaction forces.
2. Description of the Related Art
In snow, ice or other slippery conditions a driver may choose to supplement the traction of a standard tire with snow chains, or preferably a retractable automatic traction device that can be fitted within a rim and configured to extend a plurality of traction elements over the tread area of a tire. Examples of an automatic traction device are disclosed in Kahen U.S. Pat. No. 7,174,935 and Kahen US 2006/0096683, the disclosures of which are incorporated herein by this reference.
It is known that the interaction area or contact zone between the road and tire is a high stress region, with multiple tire-roadway interaction forces. These tire-roadway interaction forces can be caused by the weight and speed of the vehicle exerted on the rolling tires through rough road conditions, potholes and other changes in road conditions that can cause impact forces. The traction elements are designed to extend over portions of the tire tread, between the roadway and tread, covering the tread in spaced strips radially about the tire within this high stress region.
If the tire, with the traction elements extended thereover, encounters rough road conditions or potholes, the traction elements can be resultantly impacted and deflected due to the impact forces. If the traction element is rigidly coupled to the body of the automatic traction device, a substantial deflection could cause damage to the traction element or the traction device body, or both. Even repeated smaller deflections can cause fatigue and eventual failure over time or given life cycle. Since the traction elements are designed with a degree of stiffness to remain extended over the tread with substantial stability, they may absorb these forces well.
A large deflection could potentially damage parts of the traction device, such as the gearing, motor and other internal parts. A large enough deflection could also shear the traction element, causing it to fly off the tire with great velocity, having the potential to cause damage to cars or people. Therefore, traction device failure is undesirable not only for financial reason but also for safety reasons.
Although, the traction devices in the above-mentioned prior art references permit the traction element to extend over the tread area and selective retract off the tread area, they lack design provisions to absorb or otherwise address these tire-roadway interaction forces. Other automatic traction devices also fail to address this shortfall, such as the traction device disclosed in U.S. Pat. No. 5,540,267 to Rona. Rona's grip assemblies are rigidly coupled to the central hub, so all of the deflection is absorbed through the grip assemblies. A substantial force could easily deflect the grip assemblies to the point of breakage or damage to the other components.
Yet another shortcoming of previous attempts to mate an automatic traction device to a tire rim, such as in US 2006/0096683, is the inability to permit full retraction of the curved traction elements within the rim when the curvature of the curved traction elements is greater than the curvature of the inner diameter surface of the rim when measured cross sectionally through the central axis, the curvature being defined as the inverse of the radius. If the curvature of the curved traction element is greater than the rim's curvature, then the curved traction element's retraction will be hindered by contact with the rim. Basically, the rim does not provide the necessary clearance and the traction element does not yield. This limits the design options available for the rims and traction elements.
Accordingly, there has been a long-felt need in the art for an automatic traction device that is not adversely affected by the impacts on the traction elements due to tire-roadway interaction forces. This device should respond to the impacts with the roadway without substantial or catastrophic damage to the traction element, the device, the vehicle or nearby people. This system should also respond to impact forces from multiple directions.